Water level measurement for rivers, irrigation canals, paddy fields, water reservoirs, etc. is, in most cases, performed as follows: install a water level marker (also called a water level post) with a scale in a place where it can be seen from up close and a person visually reads a water surface position indicated by the scale. Alternatively, measurement using a so-called float type, ultrasonic type, or pressure type water level gauge is generally performed. Such a water level gauge is usually installed near the waterfront of a water area which is a measuring object and a worker reads a value indicated by the water level gauge in a site, thus checking the water level.
Now, conventionally, miniature unmanned aerial vehicles (UAVs) which are typified by unmanned helicopters for industrial use have been expensive and hard to obtain and manipulative skills have been needed to make them fly stably. However, for airframe mounted components such as an acceleration sensor and an angular velocity sensor, recently, their performance has been enhanced and their prices have decreased and UAV maneuverability has improved dramatically, because many operations to control and manipulate the UAV airframe have been automated. Against this background, attempts are now made to apply, especially, miniature multicopters to diverse missions in a wide range of fields, not only for hobby purposes.